1. Field of the Invention
This invention relates to an optical device. More particularly, it relates to devices for performing beamsplitting and/or recombination functions, and to devices employing such functions.
2. Discussion of Prior Art
The emerging field of integrated optical systems has generated a number of components analogous to those employed in electronic circuits. However, the optical signal division and recombination functions are difficult to implement in efficient, cheap and compact form. Patent Cooperation Treaty Application No PCT/GB 88/00124, (published as WO88/07179 on Sep. 22, 1988 corresponding to U.S. Pat. No. 4,975,237) relates to apparatus for light scattering measurements. It discloses use of fibre optic directional couplers for light beam division (beamsplitting) and recombination in a heterodyne light beating system. The arrangement can become complex in multiple simultaneous measurements, which require a cascaded arrangement of fibre optic couplers. It is neither inexpensive nor efficient in terms of light intensity transfer.
A particular need for multiway beamsplitting occurs in an electro-optic beamsteering device of the kind disclosed in Patent Cooperation Treaty Application No PCT/GB 88/00928 published as WO89/04988 on Jun. 1st, 1989 corresponding to U.S. Pat. No. 5,239,598. This device consists of an array of parallel waveguides of electro-optic material formed in a layered semiconductor structure. The waveguides have optical path lengths controlled by electrical bias applied to them. The array provides a common output beam which is steerable with appropriate applied bias conditions. Such a device requires light from a single source to be divided into a number of equal intensity beams (eg ten) for input to respective waveguides. This implies use of a multiway beamsplitter compatible with a layered semiconductor structure. Such a beamsplitter is not currently available.
In U.S. Pat. No. 3,832,029 and in "Image Formation Using Self-imaging Techniques", Journal of the Optical Society of America, Vol 63, No 4, Apr. 1973 pages 416-419, O Bryngdahl describes the use of a square cross-section optical tunnel to produce a self-image of a symmetrical object. The tunnel is of length L=n(2d.sup.2 /.lambda.) where n is an integer, d is the width of the tunnel and .lambda. is the wavelength of the light in the tunnel. In both references it is suggested that at lengths other than those given above multiple self images may be obtained. These lengths being L=(n+V/v)2d.sup.2 /.lambda. where V and v are integers with no common factor. However neither reference teaches what length of tunnel is necessary to achieve required numbers of self-images.
It is, however, known to employ a rectangular waveguide in a self-imaging role to provide beamsplitting, recombination and interferometric functions. This is disclosed by A Simon and R Ulrich in Applied Physics Letters, Vol 31, No. 2 Jul. 15, 1977, pages 77 to 79. The device comprises four optically polished glass blocks arranged as walls of a waveguide with rectangular cross-section. The aspect (width to height) ratio W.sub.y /W.sub.x of the waveguide is .sqroot.2; the waveguide has fully reflecting (mirror) side walls and is air cored. It has a length L=4W.sub.x.sup.2 /.lambda. (for a core refractive index of 1). The waveguide receives light from an illuminated test object at an off-centre position at one end, and generates two off-centre images at the other end for subsequent output. The off-centre input avoids output image overlap. This is the beamsplitter function. The beam combined function consists of two off-centre images at one end being transformed into two off-centre output images at the other. If the two input images are in phase then one of the two output images is dark. If the two input images are out of phase then both output images will have non zero light intensity. The beam combiner is equivalent to a beamsplitter with two input and two output ports. The interferometer function is produced by arranging two waveguides, a beamsplitter and a beam combiner, in series. The above devices are also described by R Ulrich in U.K. Patent 1 525 492, which also describes a number of other devices. These include devices employing multimode waveguides of varying cross-section to achieve magnification or demagnification.
For acceptable performance, the waveguides employed in the devices described by R Ulrich are required to support about fifty modes. This requires the waveguides to have a high core refractive index to produce high order mode confinement. Liquid-filled waveguides may be used. These are however physically cumbersome arrangements. They do not satisfy the problem of generating compact structures compatible inter alia with layered technologies such as semiconductor lithography.
The problem of dividing and recombining light beams in optical fibre systems has been addressed by A Fielding et al in the Proceedings of the European Conference on Optical Communications Gothenberg, Sweden 1989. The authors point out that tree and star optical couplers are important components for optical communications. An annular waveguide was employed to receive light output from one or more input optical fibres and to divide the light between a number of output optical fibres. The input and output fibres were disposed around the annular space of the annular waveguide. They were arranged so that the or each input fibre produced a pattern of spots centred on respective output fibre apertures. This provided sharing of the input light intensity between the output fibres. However, such annular optical systems are difficult to manufacture, since alignment requires time consuming adjustment. Here again the construction is not compatible with layer technology such as that used in semiconductor processing.
In "Passive Paths for Networks", Physics World, September 1991 pp 50-54, T Ikegami and M Kawachi review the state of the art of passive beamsplitting and recombination devices. They briefly discuss bulk devices such as beamsplitters and prisms. They move on to fibre devices such as fused fibre couplers. They state, however, that such bulk and fibre devices suffer from low productivity, lack of stability and limited suitability for optical circuit integration. They then discuss planar waveguide devices of various forms. These include a device for beamsplitting consisting of a series of Y-junctions, but again this suffers from low efficiency. They also describe an 8.times.8 star coupler consisting of eight input waveguides, twelve dummy input waveguides, a slab waveguide, eight output waveguides and twelve dummy output waveguides. It accepts eight inputs, mixes them, and distributes them evenly to the eight output waveguides. It may also accept one input and divide it equally between the eight output waveguides. The dummy waveguides are located to either side of the input and output waveguides. They are necessary in order to provide the marginal input and output waveguides with identical conditions to those located centrally. The device is fabricated on a Si substrate 5 mm.times.26 mm. It exhibits 1.42 dB average excess loss in addition to 8 dB intrinsic coupling loss. Coupling uniformity is claimed to be good with a standard deviation of 0.49 dB.
It is also known to employ complex light beam recombination/division arrangements in active optical devices such as lasers. In U.K. Patent 1 525 492 R Ulrich describes several similar laser-resonator devices. One such device consists of a length L=4W.sub.x.sup.2 /.lambda. of waveguide with plain mirrors at both ends. Mode control of the cavity may be improved by the use of small plain mirrors positioned to image one another. Their magnitude is sufficient to reflect the images formed but not to reflect spurious light. Another alternative described includes a length L/2 of waveguide with a plain mirror at one end and two small apertured plain mirrors at the other end. The apertured mirrors are positioned to image one another. These devices suffer from the same drawbacks as earlier Ulrich devices.
In Appl. Phys. Lett. 55 (19), 6th November 1989, pages 1949-51, M Jansen et al describe a laser device incorporating monolithically integrated laser diodes in a periodic array and a self-imaging Talbot cavity. This employs the Talbot effect in which a transversely disposed, periodically spaced array of light sources reimages periodically at distances which are multiples of a quantity z.sub.t ; z.sub.t is the Talbot length. It is proportional to the square of the array periodicity. It is a requirement that the array light source outputs be in phase or that adjacent outputs be in antiphase. Jansen et al disclose an array of linear, parallel diodes optically coupled to a Talbot cavity z.sub.t /2 in length and having a cleaved end surface. Reflection at the end surface produces a double pass of the Talbot cavity and an optical length of z.sub.t. This provides for the diode outputs to be reimaged on to themselves for feedback after mixing in the cavity. The cavity also filters out high order modes nearest a fundamental mode of operation. The cavity feedback strongly couples the array elements, and produces single longitudinal mode operation. It would seem that this feedback also imposes the phase relationship between the array elements which the Talbot effect requires. This arrangement is however unsuitable for single pass multiway beamsplitting, being a feedback device. Furthermore, electrical current is required in the Talbot cavity in addition to the laser waveguides. This is needed for transparency of the cavity. The device is therefore not a passive device, and is unsuitable for passive beam division and recombination applications.